Olefin copolymerization remains a significant challenge, despite the potential economic impact of incorporating of linear α-olefins (LAOs) with control over monomer sequence. It is well known in the art that single site catalyst compounds, in order to perform olefin polymerization, must be “activated”, and that these catalyst compounds are typically activated by so called “activators” which are typically aluminum or boron-based compounds and typically act as a Lewis base. EP 0 277 004 A1 is one of the first documents disclosing polymerization catalysts comprising a bis-cyclopentadienyl metallocene compound that is activated by reaction with a secondary, ionic component comprising a non-coordinating anion (NCA) and a counter-cation. Further, M. A. Giardello, M. S. Eisen, Ch. L. Stern and T. J. Marks in 117, J. Am. Chem. Soc., pp. 12114-12129 (1995), report on the preparation of cationic polymerization catalysts from metallocene compounds, using various types of activators, including methylalumoxane (MAO) and NCA/cation pairs. While these activators are required for olefin polymerization, it is desirable to not only activate the single-site catalyst, but to manipulate its activity towards, for example, comonomer incorporation, activity, etc. Traditional activators alone do not always service this end.
Also, there are known instances of catalyst-based reactivity on the timescale of polyolefin chain propagation. Epimerization of enchained propylene monomers, for example, can occur at rates sufficient to alter the tacticity of the product polymer. See Yoder, J. C.; Bercaw, J. E., 124, J. Am. Chem. Soc., pp. 2548-2555 (2002). Min, E. Y.-J.; Byers, J. A.; Bercaw, J. E., 27, Organometallics, pp. 2179-2188 (2008).
A multi-component approach to the problem or producing copolymers, referred to as “chain shuttling polymerization” requires two catalysts and a third agent to transport the growing chain between the two catalysts. See Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T., 312, Science, pp. 714-719 (2006). A single catalyst solution to this problem would simplify implementation and commercialization.
A catalyst system that interconverts between two states that possess different reactivities, can produce an oscillating distribution along a polymer chain or within a bulk polymer sample, in this case, of comonomer incorporation. The rotational dynamics of metallocene systems have been investigated over the past twenty years as a potential fluxional catalyst system; in this approach, the instantaneous symmetry of the ligand set controls the stereochemistry of propylene insertion producing stereoblock polypropylene. Lewis basic additives have been used to modify catalyst activity, but no reports exist that describe their use to control distribution of comonomer along a polymer chain. This has been achieved previously in U.S. Ser. No. 13/623,263, filed on Sep. 20, 2012, in so called “dynamic modulation” of metallocenes, where the Lewis base additive can be added during normal polymerization to modulate the reactivity of the metallocene to the monomer and comonomer. Again, a single catalyst solution to this problem would simplify implementation and commercialization. Tethered-type metallocenes are disclosed by P. W. Roesky, C. L. Stem, and T. J. Marks in 16, Organometallics, pp. 4705-4711 (1997), but does not fully address the problem of producing ethylene-based block copolymers.
There is still a need in the art for new and improved catalyst systems for the polymerization of olefins, in order to achieve specific polymer properties, such as high melting point, high molecular weights, to increase conversion or comonomer incorporation, or to alter comonomer distribution without deteriorating the resulting polymer's properties. It is therefore an object of the present invention to provide a process and a catalyst system for use in a process for the polymerization of olefins, especially ethylene, wherein the resulting polymers have good melting points and comonomer distribution. Ideally, such catalyst system should also exhibit high catalytic activity under commercial polymerization conditions.
In another embodiment, it is an object of the present invention to provide a process and a catalyst system for use in a process for the polymerization of olefins, especially ethylene, to make block copolymers.
Additional references of interest include F. Amor et al.: Journal of Organometallic Chemistry 558 (1998) 139-146; WO 1997/14724; Organometallics 1998, 17, 5836-5849; Organometallics 1995, 14, 3129-3131; and Journal of Organometallic Chemistry 591 (1999) 127-137.